1. Field of the Invention
The present invention relates to the field of optical sensors associated with semiconductor devices and, more specifically, to the manufacturing of a semiconductor package with an optical sensor and especially of the optical unit thereof.
Such semiconductor packages with an optical sensor are used, for example, to form miniature cameras or shooting devices, for example in portable phones.
2. Discussion of the Related Art
Optical units for devices with an optical sensor of semiconductor packages are essentially formed of a hemispherical lens arranged on a diaphragm placed at a focal distance from a microlens array formed on an integrated circuit chip forming the optical sensor.
A first known family of optical units is formed of a rotatable plastic cap intended to hang up the lens and the diaphragm, and to be arranged on the integrated circuit chip. The cap rotation is especially used to adjust the focal distance, and thus as a focusing device. This optical unit family is generally intended for lenses made of plastic material having positioning and manufacturing tolerances (on the order of more or less 80 micrometers) incompatible with the small focal distance (a few millimeters) of this type of device.
A second known family of optical units consists of using a glass lens (generally hemispherical) arranged on a diaphragm formed on a glass spacer, itself resting on the integrated circuit chip supporting the microsensors (phototransistors, microdiaphragms, and microlenses). The present invention more specifically applies to this family of optical units.
FIG. 1 shows a conventional example of a semiconductor package with an optical sensor such as described in French patent application 03/12305 (03/GR2/190) which is incorporated herein by reference.
Such a package 10 forming a shooting element comprises an integrated circuit chip 1 forming an optical sensor, on which hemispherical microlenses 2 have been shown. In practice, these microlenses are integrated in the silicon of chip 1 above the phototransistors.
Package 10 also comprises a lens 3 arranged at a distance from chip 1 and a diaphragm 4 against the lower surface (opposite to the sensor) of lens 3. A glass spacer 6, relatively thick (on the order of a few millimeters) with respect to the thickness of chip 1, is interposed between chip 1 and diaphragm 4 and defines the focal distance of the optical device. The above-described assembly is formed by batches in whole plates before being cut for individualization. The assembly then rests on a substrate 5. The electric information provided by integrated circuit chip 1 is transferred, for example, from the upper surface of this chip supporting microlenses 2 by means of wires 52 of contact transfer to pads 53 at the upper substrate surface, pads 53 being electrically connected to pads 54 at the rear (free) surface of substrate 5 via vias 51 across its thickness. Pads 54 then generally receive conductive balls or bosses 55 for assembly of the semiconductor package on a printed circuit, as a BGA (ball grid array) package.
In practice, spacer 6 is obtained by means of a glass plate against the respective surfaces of which are arranged on the one hand a diaphragm grid 4 and a lens plate 3 and, on the other hand, integrated circuit chips 1 supported by substrates 5, a spacer 7 of relatively small thickness with respect to the thickness of spacer 6 being provided to avoid for chip 1 to touch spacer 6 and to be damaged on assembly.
In the example of FIG. 1, it is assumed that optics 3 are formed of a hemispherical glass lens. As an alternative, the definitive curvature of the focusing lens may be adapted by plunging the hemispherical lens into a cavity containing an optical resin which polymerizes by a replication method, to add a surface layer (not shown) giving it the desired curvature.
Once the devices, formed of chip 1 supported by substrate 5, of spacer 6, and of optics 4, have been individualized, a casing 8 generally made of plastic matter (resin) is arranged around the assembly to protect the system and to complete package 10.
FIGS. 2A to 2D illustrate the conventional assembly of hemispherical lens 3 on diaphragm 4, supported by glass spacer 6. This assembly is generally performed after cutting of integrated circuits 1 and of spacers 6 supporting diaphragms 4.
A first step (FIG. 2A) consists of depositing a drop g of glue on diaphragm 4 supported by spacer 6.
The glue spreads on the diaphragm (FIG. 2B) and fills opening 41 thereof. A layer g′ of glue is obtained on the structure.
A hemispherical glass lens 3 is then added (FIG. 2C) on glue layer g′ and the assembly is submitted to an ultraviolet processing to have the glue polymerize. The glue is selected to be thixotropic, that is, with no capillary effect, to avoid its upward migration on the lens, which would modify its optical properties.
Finally (FIG. 2D), integrated circuit 1 is arranged on the rear surface of glass spacer 6, conductive bosses 54 are formed at the rear surface and the entire unit is topped with plastic matter to form casing 8.
A problem of the assembly thus formed is that, when lens 3′ is laid on non-solidified glue layer g′, it floats thereon, which considerably increases horizontal positioning tolerances. This phenomenon is particularly notable when the lenses are desired to be assembled on whole wafers. A disadvantage is that this generates resolution problems.
In certain cases, diaphragms 4 are formed of a metal grid arranged on the glass wafer forming spacers 6. In such a case, an additional problem of horizontal shift of the metal gates with respect to the centers of the lenses appears. Further, the etch tolerances of openings 41 in these grids are greater (on the order of more or less 5 micrometers) with respect to the tolerances in the lens diameter (more or less 3 micrometers for a diameter on the order of 3 millimeters).
The lens floating on the glue mat however remains the most critical since it adds a much greater tolerance, on the order of more or less 100 micrometers.